Hitherto, a buildup multi-layer printed circuit board has been manufactured by a method disclosed by, for example, Japanese Patent Laid-Open No. 9-130050.
A rough layer is formed on the surface of the conductor circuit of a printed circuit board by electroless plating or etching. Then, an interlayer insulating resin is applied, exposed and developed by a roll coater or printing, via hole opening portions are formed for making layers continuous, and an interlayer resin insulating layer is formed through UV hardening, actual hardening and the like. Further, a catalyst such as palladium is applied on to the interlayer resin insulating layer on the rough surface which has been subjected to a roughing process with an acid or an oxidizer. A thin electroless plated film is formed, a pattern is formed on the plated film by a dry film and the thickness of the pattern is increased by electroplating. Thereafter, the dry film is separated and removed by an alkali and etched to thereby form a conductor circuit. By repeating the above processes, a buildup multi-layer printed circuit board is obtained.
At present, as the frequency of IC chips becomes higher, demand for accelerating the transmission speed of a multi-layer printed circuit board rises. To deal with such demand, the applicant of the present invention proposed Japanese Patent Laid-Open No. 10-334499. With this constitution, linear wirings are provided by arranging via holes 346 of a lower interlayer resin insulating layer 350 and via holes 366 of an upper interlayer resin insulating layer 360 right above through holes 336, thereby shortening wiring lengths and accelerating signal transmission speed.
It was discovered, however, that with the above constitution, the via holes 346 of the lower interlayer resin insulating layer 350 and the via holes 366 of the upper interlayer resin insulating layer 360 are separated from one another under heat cycle conditions. The inventor of the present invention investigated the cause of separation and discovered that the via holes 366 in the upper layer are influenced by the shapes of the surfaces of the via holes 346 of the lower layer and the connection characteristic of the via holes 366 deteriorates. Further, it is estimated that since the interlayer resin insulating layers 350 and 360 are not reinforced by core materials such as glass cloth, these layers tend to be separated in a heat cycle rather than a core substrate provided with a core material.
The present invention has been made to overcome the foregoing problems, and it is, therefore, an object of the present invention to provide a multi-layer printed circuit board and a method of manufacturing a multi-layer printed circuit board capable of shortening internal wiring lengths and having excellent connection reliability.
It is a still further object of the present invention to provide a manufacturing method capable of manufacturing a multi-layer printed circuit bard at low cost.
Meanwhile, a resin is filled in through holes so as to enhance reliability for a buildup multi-layer printed circuit board. When filling the resin, blackening-reduction processes are conducted to the surfaces of the through holes and rough layers are provided thereon so as to increase adhesiveness. In addition, as the density of the multi-layer printed circuit board increases, through holes are made smaller in size. Following this, resin filler having low viscosity is employed to be filled in the through holes.
As prior art for forming a rough layer on a through hole and filling the through hole with resin filler, it is described in Japanese Patent Laid-Open No. 9-181415 that a copper oxide layer is formed in a through hole, the through hole is filled with resin filler and then an interlayer insulating layer is formed. It is also described in Japanese Patent Laid-Open No. 9-260849 that after forming a rough layer in a through hole by etching, the through hole is filled with resin filler and then an interlayer insulating layer is formed.
If using resin filler having low viscosity, however, the resin filler is dented in the through hole, causing disconnection and the like during the formation of wirings on an upper layer. The inventor of the present invention investigated the cause of disconnection and discovered that this is because the resin out of filler and the resin which constitute resin filler flow along the rough layer (very small anchor) formed on the land of the through hole. As a result, the filler within the through hole is dented, making it impossible to flatten and smooth a core substrate. Due to this, it was discovered that if manufacturing a multi-layer printed circuit board by forming an interlayer resin insulating layer and wirings on a core substrate, the resultant multi-layer resin insulating layer is susceptible to disconnection and a probability of generating defects increases.
The present invention has been made to solve the foregoing problems and it is, therefore, a still further object of the present invention to provide a method of manufacturing a multi-layer printed circuit board having enhanced wiring reliability.
In the meantime, a substrate on which a resin film for the interlayer resin insulating layer of a resin substrate serving as a core material is bonded, is employed as a core substrate. Through holes for penetrating the substrate are filled with resin filler. Further, an interlayer resin insulating layer is formed and via holes are formed therein. The above-stated resin filler, however, had some defects.
First, if a reliability test such as a heat cycle is conducted to a printed circuit board filled with filler, conductors sometimes crack in the vicinity of the boundary between the resin substrate and the resin film. Second, after filling the filler, a resin film serving as an interlayer resin insulating layer cracks in a polishing step conducted to flatten the board. Third, if a plated cover is formed right on the through hole, the reaction of the plated film may stop. Thus, even if via holes are formed right above the through holes, electrical connection cannot be established.
As a result of these three defects, a printed circuit board with deteriorated reliability and reduced electrical connection characteristics is provided.
It is a still further object of the present invention to provide a printed circuit board and a method of manufacturing a printed circuit board capable of solving these defects.